Copper-mediated reduction of CO2 with pinB-SiMe2Ph via CO2 insertion into a copper-silicon bond

Article abstract of DOI:10.1021/ja208969d

Reaction of [(IPr)Cu-OtBu] (1) with pinB-SiMe₂Ph (2) leads to the Cu-silyl complex [(IPr)Cu-SiMe₂Ph] (3). Insertion of CO ₂ into the Cu-Si bond of 3 is followed by transformation of the resulting silanecarboxy complex [(IP

Full text of DOI:10.1021/ja208969d

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Copper-Mediated Reduction of CO2 with pinB-SiMe2Ph via CO2
Insertion into a CopperÀSilicon Bond
Christian Kleeberg,*,† Man Sing Cheung,‡ Zhenyang Lin,‡ and Todd B. Marder§
†Institut f€ur Anorganische und Analytische Chemie, Technische Universit€at Carolo-Wilhelmina zu Braunschweig, Hagenring 30,
38106 Braunschweig, Germany
‡Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong (China)
§Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
S Supporting Information
b
Scheme 1. Simplified Mechanism of the Copper-Catalyzed
ABSTRACT: Reaction of [(IPr)Cu-OtBu] (1) with pinB-
SiMe2Ph (2) leads to the Cu-silyl complex [(IPr)Cu-
SiMe2Ph] (3). Insertion of CO2 into the CuÀSi bond of
3 is followed by transformation of the resulting silanecar-
boxy complex [(IPr)Cu-O2CSiMe2Ph] (4) to the silanolate
complex [(IPr)Cu-OSiMe2Ph] (5) via extrusion of CO. As
5 reacts readily with 2 to regenerate 3, a catalytic CO2
reduction to CO is feasible. The individual steps were
studied by in situ 13C NMR spectroscopy of a series of
stoichiometric reactions. Complexes 3, 4, and 5 were
isolated and fully characterized, including single-crystal
X-ray diffraction studies. Interestingly, the catalytic reduc-
tion of CO2 using silylborane 2 as a stoichiometric redu-
cing agent leads not only to CO and pinB-O-SiMe2Ph
but also to PhMe2Si-CO2-SiMe2Ph as an additional reduc-
tion product.
Reduction of CO2 to CO with B2pin2 According to DFT
Calculations5
The corresponding reaction for E = Bpin has been studied
in detail both experimentally3a and by DFT calculations
(Scheme 1).5 For E = Bpin the formation of [(NHC)Cu-
OBpin)] along with the reduction of CO2 to CO is observed.3a
This complex reacts with B2pin2, regenerating the boryl complex
and producing (pinB)2O. Hence, the overall process is the Cu-
catalyzed reduction of CO2 to CO by B2pin2. The thermodyna-
mically crucial step in CO2 reduction to CO is the cleavage of the
very stable OdC double bond (532 kJ molÀ1).6 The aforemen-
tioned Cu-catalyzed process overcomes this by forming BÀO
bonds (806 kJ molÀ1),6 providing the thermodynamic driving
force for the process.
As the SiÀO bond is thermodynamically comparable (799 kJ
molÀ1),6 an analogous reaction mechanism involving a silyl
copper complex can be envisaged. The copper alcoholato com-
plex [(IPr)CuOtBu] (1) is known to undergo transmetalation
with B2pin2 and with (OCH2CMe2CH2O)B-(4-MeOC6H4) via
a σ-bond metathesis pathway to form the corresponding boryl or
aryl complex, respectively.3a,d,5,7 The related reaction with pinB-
SiMe2Ph is suggested to occur in the recently reported, Cu-
catalyzed silyl(bor)ation reactions.1 Indeed, [(IPr)CuSiMe2Ph]
(3) is formed quantitatively (NMR)8 via reaction of 1 with pinB-
SiMe2Ph (2) under mild conditions and can be isolated in 89% yield.
The selectivity of this reaction, hence, formation of the Cu silyl
opper-catalyzed silylation reactions employing silylboranes
C
as silyl sources have recently attracted considerable
attention.1 It has been proposed that in these reactions an
electrophilic substrate (e.g., α,β-unsaturated carbonyl com-
pound, imine, allyl chloride) reacts with a nucleophilic copper
silyl species to form a copper γ-silylalcoholato, α-silylaminato, or
halo complex. From this complex the copper silyl species is
regenerated by a transmetalation reaction with a silylborane. This
is believed to proceed either directly, in the case of copper γ-
silylalcoholato complexes, or after formation of a copper alco-
holato complex, in the case of α-silylaminato and halo complexes.1,2
A recently published synthetic and mechanistic study on the 1,
2-silylboration of aldehydes supports this general mechanism.
However, for kinetic reasons the transmetalation step itself,
hence the regeneration of the copper silyl species, could not be
studied in the particular system used.2 In this study, we examined
the reaction of a (IPr)Cu silyl complex with CO2 as an exemplary
electrophile.
The reaction of copper complexes of the type [(NHC)Cu-E]
(NHC = N-heterocyclic carbene) with CO2 has been studied for
E = B((OC(CH3)2)2) = Bpin, SnPh3, CH3, 4-(CH3O)C6H4),3
but for E = SiPh3, the reaction has only been noted briefly to give
[(IPr)CuOSiPh3] (IPr = 1,3-bis(2,6-di-isopropyl-phenyl)imidazol-
2-ylidene).3b In agreement with this report is a recently published
DFT study suggesting that the initially formed complex [(IPr)-
Cu-O2C-SiPh3] releases of CO to give [(IPr)CuOSiPh3].4
Received: September 23, 2011
Published: November 09, 2011
r
2011 American Chemical Society
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Figure 1. Molecular structure of 3. Selected bond lengths [Å] and angles
[°]: Cu1ÀC1 1.933(1), Cu1ÀSi1 2.2784(4), C1ÀCu1ÀSi1 170.53(4).8
Figure 3. Molecular structures of 4 and 5. Selected bond lengths [Å]
and angles [°]: for 4 (left), Cu1ÀC1 1.861(2), Cu1ÀO1 1.844(2),
O1ÀC28 1.282(3), O2ÀC28 1.237(3), C1ÀCu1ÀO1 173.6(1); for 5
(right), Cu1ÀC1 1.854(2), Cu1ÀO1 1.797(2), O1ÀSi1 1.602(2),
C1ÀCu1ÀO1 179.41(8).8
clean conversion to 5 with release of CO.8 The fast insertion of
CO2 into the CuÀSi bond is in agreement with the calculated
barriers for this process of 27.0 and 22.3 kcal molÀ1, respectively,
for two different model systems.4,8 The formation of the silane-
carboxylate complex 4 instead of the isomeric complex [(IPr)Cu-
CdO(OSiMe2Ph)] is attributed to the high kinetic nucleophi-
licity of the silyl moiety in 3, in contrast to its high thermo-
dynamic oxophilicity.5,10 The insertion product 4 releases CO
with formation of the silanolate complex 5. The formation of 5 as
the final product is in agreement with the report by Sadighi et al.
and the related Cu-boryl system.3a,b,5 Computational data on this
step support a higher barrier of activation than for the initial
insertion step of 24.6 (SiPh3) and 31.3 kcal molÀ1 (SiMe2Ph),
respectively, rendering this step rate-determining in the overall
reaction.4,8 The calculated barrier for the related Cu-boryl system
is lower (22.0 kcal molÀ1), rationalizing the fact that no inter-
mediate carboxyl complex was observed for this system.3a,5
The above results and close similarities to the CO2-reducing
Cu-boryl system of Sadighi et al. suggest that 3 should catalyze
the reduction of CO2 to CO using 2 as a stoichiometric reducing
agent.3a The only additional reaction necessary in a proposed
catalytic cycle analogous to Scheme 1, but with 2 instead of
B2pin2, is the regeneration of 3 from the silanolate complex 5 and
2. This reaction is related to the formation of 3 from 2 and
[(IPr)Cu-OtBu] and to the reported formation of [(IPr)Cu-
Bpin] from [(IPr)Cu-OBpin] and B2pin2.3a Monitoring the
reaction of 5 with pinB-SiMe2Ph revealed that, indeed, 3 is
formed cleanly along with pinB-O-SiMe2Ph (6) (Figure 2, top).8
The reaction was performed with 5 prepared in situ after ex-
changing the 13CO2 atmosphere for dinitrogen. The presence of
residual dissolved 13CO2 gives rise to the formation of traces of 4,
illustrating the viability of the proposed catalytic reduction. DFT
calculations predicted an activation barrier of 22.0 kcal molÀ1 for
this transmetalation step, significantly higher than that for the
related step in the Cu-boryl system (14.3 kcal molÀ1).5,8
Figure 2. Time-dependent in situ 13C{1H} NMR spectra of the reaction
of 3 with 13CO2 in C6D6 at ambient temperature (O = unidentified
signal).
rather than boryl complex, can be rationalized by the ease of
quaternization of the Bpin moiety compared to the more difficult
increase in coordination number from four to five at the silicon atom.9
Linear complex 3 was fully characterized, including a single-
crystal X-ray structure determination (Figure 1).8 The silyl group
in 3 has, in agreement with computational data, a trans-influence
similar to that of the Bpin moiety and significantly stronger than
that of anionic tin and carbon ligands.3h
The reaction of 3 with excess 13CO2 was monitored by 1H and
13C NMR spectroscopy (Figure 2).8 After 1 h the complete
consumption of 3 (signal of the SiMe2 moiety at 4.3 ppm,
carbene C signal at 185.7 ppm) and the exclusive formation of
two new species were observed. A signal at 191.1 ppm with Si
satellites (1JCSi = 91 Hz, 13C-enriched) indicates a carbonyl C
atom next to a Si atom. This signal is assigned, together with a
doublet at À2.2 ppm (2JCC = 4 Hz, SiMe2), to compound 4.
Additionally, a singlet at 4.0 ppm (SiMe2), indicative for 5, and a
signal indicative for free CO at 184.4 ppm are observed. After 6 h,
the signals at 191.1 and À2.2 ppm have nearly vanished while the
signal at 4.0 ppm has increased; after 22 h, only the signal at 4.0
ppm is observed. Hence, the 13C NMR data suggest the fast
initial formation of 4 from 3 and CO2 followed by a slow but
Complex 4 was initially obtained as several single crystals from
a solution of 3 in the presence of traces of CO2. More
reproducible and suited for the preparation of larger quantities
is the reaction of 3 with a 1:1 mixture of CO and CO2 and
crystallization of 4 under a CO atmosphere (29% yield). It may
be that a CO adduct of 4 inhibits its decomposition to 5. The
dependence of the stability of 4 on the presence of CO is a
subject of ongoing computational and kinetic studies. Complex 5
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Figure 4. Detail of time-dependent IR spectra during the reduction of
CO2 to CO with 2 in the presence of catalytic amounts of 3. Inset:
Conversion of CO2 to CO (O, amount of CO2; b, amount of CO).8
Figure 6. Molecular structure of 8; the aromatic rings are represented
by the ipso-carbon atom only (spheres). Selected bond lengths [Å] and
angles [°]: Cu1ÀC1 1.869(7), Cu1ÀO1 1.988(4), Cu1ÀO3 2.248(4),
Cu1ÀO5 2.315(5), C1ÀCu1ÀO1 151.3(3), C1ÀCu1ÀO3 116.3(2),
C1ÀCu1ÀO5 119.0(3).8
Figure 5. In situ 13C{1H} NMR spectrum of the reduction of 13CO2
with 2 in the presence of a catalytic amount of 3 after complete
consumption of 2 (79 h).
was readily isolated in 79% yield by crystallization from the
reaction mixture of 3 and excess CO2 (Figure 3).8
The reaction of excess 2 (220 μmol) with CO2 (98 μmol) in
the presence of a catalytic amount of 3 (19 μmol, 20 mol %)8 was
monitored in situ by IR spectroscopy of the gas phase above the
reaction mixture,8 which showed that CO2 is converted slowly,
but catalytically, to CO (Figure 4). However, complete con-
sumption of the CO2 results in only 70% conversion to CO in the
gas phase (Figure 4).
Figure 7. 13C{1H} NMR spectra of 8 after 2 h (bottom) and after 16 h
(top) at ambient temperature.
Reaction of 2 with a ca. 35% excess of 13CO2 in the presence of
10 mol % of 3 was monitored by in situ 13C NMR spectroscopy.
Complete consumption of 2 requires 79 h at ambient tempera-
ture and pressure (Figure 5).8,11 The expected products CO and
pinB-O-SiMe2Ph (6) were identified by their distinctive 13C
NMR signals (CO, 184.4 ppm; 6, 0.1 ppm (SiMe2)). However,
additional signals at 187.1 (s), À4.1 (d, 2JCC = 4.8 Hz), and À0.9
ppm (s) were assigned to Me2PhSiCO2-SiMe2Ph (7) by com-
parison with an authentic sample. Additionally, the presence of
(Bpin)2O was detected.8 Signals at ca. À3.8 (two signals, one
broadened) and 185 ppm (broadened) have not yet been
assigned but arise from species containing a carbonyl and a
SiMe2 moiety.8 During the catalysis, a single set of signals
characteristic for iPr moieties is present, suggesting that there
is one Cu complex present in detectable amounts and that it is
not 3, 4, or 5.8 This indicates that the catalytic process is more
complicated than expected from the stoichiometric reactions. In
accord with this, crystals grown at low temperature from the
catalytic reaction before completion proved to be Cu complex 8
(Figure 6), which was also characterized by NMR spectroscopy.8
Complex 8 can be described as the Lewis acid/base adduct of 4
and pinB-O2C-SiMe2Ph (9). The formation of 9 suggests that
the transmetalation of a SiMe2Ph moiety from 2 can occur not
only by reaction of 5 with 2 to give 3 and 6, but also by reaction of
4 with 2 to give 3 and 9. However, neither 8 nor 9 was observed
by NMR spectroscopy during the catalytic reduction. This may
be explained by the low stability of 8 in solution (Figure 7).
1H, 13C, and 11B NMR spectra obtained from a freshly
prepared solutions of 8 indicate the presence of essentially one
species (Figure 7, bottom), consistent with the molecular structure
1
of 8. The broad SiMe2 H NMR signal (À3.0 ppm, Δw1/2 = 11 Hz)
suggests a dynamic process. Additionally, traces of 6 (0.1 ppm,
Δw1/2 = 2 Hz) indicate some decomposition of 8. After 16 h, the
13C NMR spectrum dramatically changed; 4, 6, and 7 are unam-
biguously identified by the distinct 13C resonances (4, 191.1
(CdO), 182.2 (CÀCu), À2.2 ppm (Me2Si); 6, 0.05ppm(Me2Si);
7, 187.1 (CdO), À0.9 (Me2SiO), À4.1 ppm (Me2SiCO2)),
and (pinB)2O was detected.8 The formation of these species
can be rationalized by fragmentation of 8 into 4 and 9 along
with the decomposition of 9 into 6 and 7. The formation of 7
and 6 from 9 along with the loss of CO is in agreement with the
known reactivity of related silanecarboxylic acid derivatives.12
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Scheme 2
’ AUTHOR INFORMATION
Corresponding Author
ch.kleeberg@tu-braunschweig.de
’ ACKNOWLEDGMENT
Support for this research by a Liebig-Stipendium of the Fonds
der Chemischen Industrie for C.K. is gratefully acknowledged.
Initial work conducted at Durham University was supported by a
DFG Fellowship to C.K. T.B.M. thanks the Royal Society for a
Wolfson Research Merit Award, the Alexander von Humboldt
Foundation for a Research Award, and EPSRC for an Overseas
Research Travel Grant. Z.L. thanks the Research Grants Council
of Hong Kong for support (HKUST 603711).
Scheme 3. Proposed Catalytic Cycle for the Cu-Catalyzed
Reduction of CO2 to CO with 2
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The stoichiometry of the decomposition of 9 can be rationalized
by the equation given in Scheme 2. While the mechanism of this
reactions is the subject of ongoing research, it may be related to
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ment of α-silylalcohols.12
(4) Ariafard, A.; Brookes, N. J.; Stranger, R.; Yates, B. F. Organome-
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It appears that the higher stability of the silanecarboxylic acid
complex 4 compared to the related species in the Cu boryl system
and its ability to undergo transmetalation with 2 forming 9 are
crucial for the distinct reactivity observed here.3a,b,5 Plausible
pathways relevant to the Cu-catalyzed reduction of CO2 to CO
with 2 are shown in Scheme 3.
In conclusion, the stoichiometric reduction of CO2 to CO
occurs via a two-step process. The insertion of CO2 into a CuÀSi
bond of complex 3 leads to formation of the isolable silanecar-
boxylato complex 4, which slowly forms the silanolate complex 5,
releasing CO. However, in situ NMR spectroscopy during
catalysis suggests a more complicated mechanism than postu-
lated on the basis of the stoichiometric reactions, involving at
least the reaction of 4 with 2, forming 9 and, presumably, 3.
Detailed mechanistic and DFT studies are ongoing.
(5) Zhao, H.; Lin, Z.; Marder, T. B. J. Am. Chem. Soc. 2006, 128,
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(8) See SI for details.
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(11) It was also verified that no reaction occurs between 2 and CO2
in the absence of the copper complex as well as in the presence of CuCl
under otherwise identical conditions. See SI for details.
(12) The implied instability of 9 is also in agreement with the
observation that the reaction of pinBH with Me2PhSiCO2H leads to 7
and not 9. While the related reactions of pinBH with Me2PhSiOH and of
Me2PhSiCl with Me2PhSiCO2H lead to 6 and 7, respectively. See SI for
details. For the stability of silanecarboxylic acid derivatives and related
compounds, see: (a) Igawa, K.; Kokan, N.; Tomooka, K. Angew. Chem.,
Int. Ed. 2010, 49, 728–731. (b) Brook, A. G. J. Am. Chem. Soc. 1955,
77, 4827–4829. (c) Brook, A. G. Acc. Chem. Res. 1974, 7, 77–84.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures, ana-
b
lytical and crystallographic data, NMR spectra, and details of
DFT calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
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